This invention relates to a method and apparatus for changing the spectral content of a data stream suitable for high-speed local area network communications over wire lines. It is contemplated that serial information is to be transmitted at extremely high data rates over specific media and relatively short distances (up to 100 meters on data grade unshielded twisted pair cabling or level 5 cable, and up to 200 meters on Type 1 shielded cabling). The particular field of application is in communications according to the Fiber Distributed Data Interface (FDDI) standard.
FDDI is a high-speed LAN protocol based on use of a token in a ring topology and originally designed only for fiber optic networks communicating at video data rates. According to the FDDI Physical Media Dependent (PMD) layer protocol encoding is Non Return to Zero Invert (NRZI) with a 4-bit to 5-bit conversion/translation which guarantees no more than a 14% deviation from a reference d.c. level. Data rates are such that unwanted spurious emissions may be generated which cause electrical interference.
FDDI allows two types of network stations or nodes on an electrically-connected ring. One class, Class A, may couple to a primary and a redundant secondary ring simultaneously. Another class, Class B, may be coupled to only one ring at a time. Class B nodes are intended to use a single connection to carry both incoming and outgoing lines of a ring. However, because of the dual fiber configuration of the primary ring, Class A nodes and Class B nodes can be interconnected only through an interfacing device referred to as a wiring concentrator. The wiring concentrator provides connection points into the primary ring that are suitable for Class B type connections.
The connection between Class B nodes and the wiring concentrator may be optical fiber or copper twisted pair wire, such as IBM Type 1 shielded wire. It would be advantageous to use Data Grade high twist cabling or conventional telephone DIW unshielded twisted pair cabling, particularly in a building structure already wired with such wiring. However, it has been considered difficult, if not impossible, to use such wiring at data rates comparable to the fiber optic speeds of interest. As a consequence, there is a constraint on the rate of data exchange which prevents the use of such wiring in the connection between a Class B node and a data concentrator. It turns out that data rates of 125 MB/s can be supported by twisted pair wiring, provided that RF emissions can be reduced to acceptable levels. FCC standards require that emission levels be suppressed in the spectrum of 30 MHz to 1 GHz. Current FDDI modulation schemes cannot meet these constraints on unshielded twisted pair wiring.
Data scramblers are known for the purpose of encrypting data for secure transmission or to provide for error detection and correction in noisy communication channels. Some incidental spectral modification generally results. However, such spectral shaping is generally a side effect which typically results in a degraded spectrum. A primitive scrambling and descrambling technique, NRZ/NRZI encoding, is part of the FDDI specification. The specified scrambling technique aggravates the emissions problem by producing, especially in response to the FDDI "idle" signal, a strong square-wave frequency characteristic with a fundamental frequency of 62.5 MHz and strong odd harmonics. The FDDI "idle" symbol is 11111. In between frames of data, the "idle" signal is transmitted to keep the receive clocks at each "listening" station synchronized to the transmit clocks at each "talking" station. After NRZI encoding, this idle signal becomes 10101--a 62.5 MHz square wave.
What is needed is a scheme for communication of baseband signals on copper wiring which is capable of data rates comparable to that of fiber optic cables while satisfying the stated constraints.